alpha2-Macroglobulin (alpha2M) is a major inhibitor of virtually all known endoproteases found in the plasma of vertebrates. It has been suggested that it serves as a general protease scavenger, thereby protecting the blood and tissue proteins from degradation. As a protease inhibitor, it may play a role in the regulation of protease activity in fibrinolysis, coagulation, and complement activation. As a result, alpha2M is a large glycoprotein (Mr=725,000) composed of four identical subunits. Protease inhibition is accompanied by a major change in its structure resulting in entrapment of the protease. In order to determine the mechanism by which the native structure rearranges to the protease-bound form, immunoelectron microscopy will be employed. The structures will be visualized using both negative-stain and cryoelectron microscopy and maximum resolution of these micrographs will be achieved by image processing. Immunoelectron microscopy and gold cluster labeling will also be utilized to locate the sites of biological interest, which include the "bait" region, the receptor-binding sites, and the thioesters. Further details of the structural consequences of protease binding will be studied by electron microscopy of the methylamine-treated and the alpha2M-protease complexes with one and two molecules of trypsin bound.